1110 ISSN 0030-400X, Optics and Spectroscopy, 2019, Vol. 127, No. 6, pp. 1110–1116. © Pleiades Publishing, Ltd., 2019. Stability of Optical Responses from Lead-free Perovskite Films E. V. Ushakova a, b, *, A. I. Matuhina a , A. V. Sokolova a , S. A. Cherevkov a , K. V. Bogdanov a , A. Dubavik a , M. A. Baranov a , A. P. Litvin a , K. Takai c , A. V. Fedorov a , and A. V. Baranov a a ITMO University, St. Petersburg, 197101 Russia b City University of Hong Kong, Hong Kong SAR c Hosei University, Tokio, 1848584 Japan *e-mail: elena.ushakova@itmo.ru Received April 24, 2019; revised April 24, 2019; accepted September 9, 2019 Abstract—Materials with perovskite-type crystal structure attract much attention due to their unique optical properties, such as high quantum yield of photoluminescence and high carrier mobility. However, their fur- ther utilization as an active media in photonic and optoelectronic devices is limited by the lack of stability of their optical responses together with the presence of lead cations in the chemical composition. Here, a detailed study of the optical properties and their stability under ambient conditions of perovskite films with the chemical composition (CH 3 NH 3 ) x A y I z , where A = Pb, Bi, Sb, was performed. The changes in optical responses of the samples were monitored for 3 months. It was shown that the stability of perovskite films was improved by surface passivation with a thin layer of polymethylmethacrylate onto their surface. The devel- oped procedures of film formation and following surface passivation are of interest in the design of active opti- cal materials based on perovskite films with the increased optical performance. Keywords: Raman spectra, photoluminescence, bismuth, antimony, PMMA DOI: 10.1134/S0030400X19120270 INTRODUCTION The design and development of novel optical mate- rials are one of the main directions of modern Material Science. The synthesis and investigation of the optical properties of materials with a perovskite-type crystal structure are of both fundamental and applied scien- tific interest. These materials attract increasing atten- tion due to such unique properties as high carrier mobility (caused by a high diffusion length of ~1 μm [1]), high exciton binding energy (~50–60 meV, allowing exciton transitions to be observed even at room temperature [2, 3]), a high photoluminescence (PL) quantum yield [4–8]. In particular, materials with perovskite symmetry are in demand for solar energy applications. To date, perovskite-based solar cells have already achieved a conversion efficiency of 23.3% [9]. They are also in demand as an active medium for laser radiation sources. Due to their min- iature size and the ability to tune the emission wave- length by modifying their chemical composition which allows them to cover the entire visible spectrum, they have great potential for integration into optoelec- tronic systems [10–17]. Despite the obvious perspective of these materials, there are a number of problems that need to be tackled before these technologies are commercialized. One of these problems is the presence of toxic lead cations in the perovskite chemical composition. A possible solu- tion is to use elements from the 14th group of the peri- odic table instead of lead atoms, for example, tin (Sn) and germanium (Ge) [18]. However, rapid atmo- spheric degradation of such perovskite material upon exposure to oxygen and water vapor, i.e. the oxidation of Sn 2+ cations to Sn 4+ , does not allow achieving the high stability performance under ambient [19–26]. Perovskite materials based on Ge cations have a simi- lar tendency to oxidize to Ge 2+ [27, 28]. Other divalent cations, for example, Sr and Ba, are also used as a sub- stituting element for lead cation [29, 30]; however, these materials have a large band gap which signifi- cantly limits their applications. The perovskite-type structure is also typical of ferroelectromagnets such as BiFeO 3 , BiFe 2 CrO 6 , and BiMnO 3 [31–35] which makes it possible to consider elements of the 15th group, such as bismuth (Bi) and antimony (Sb), as a potential replacement for Pb atoms. Taking into account the abovementioned issues, the development of protocols for the formation of lead-free perovskites and the study of their optical properties are the priority tasks for the development of this field of photovoltaics. Here, we investigated the optical responses of lead-free perovskite thin films in comparison with lead-based perovskite films in detail. The efficient passivation of the film surface can be achieved by the using of the thin polymer layer depo- OPTICS OF LOW-DIMENSIONAL STRUCTURES, MESOSTRUCTURES, AND METAMATERIALS